1. Field of the Invention
The present invention relates to image scanning devices and document transporting methods, and in particular to an improvement of an image scanning device that consecutively supplies and transports at least two documents.
2. Description of the Related Art
Image scanning devices capable of automatically transporting two or more documents and sequentially scanning the documents are widely used. Further, image scanning devices capable of carrying out double-side scanning of a document by automatic transportation are also widely used. As types of such image scanning devices capable of carrying out double-side scanning, a 1-pass-2-scanner method and a 2-pass-1-scanner method are known. The 1-pass-2-scanner method carries out the double-side scanning of a document using two image sensors. The 2-pass-1-scanner method carries out the double-side scanning of a document using a single image sensor and turning sides of the document during the automatic transportation.
A switching back technique and a U-turn technique are known as the double-side scanning by the 2-pass-1-scanner method. According to the switching back technique, a document is caused to pass through a single image sensor twice in a same direction by carrying out a switch-back for switching a transportation direction during the transportation of the document. According to the U-turn technique in which the transportation paths are crossed using a U-turn path, a document is caused to pass through a single image sensor twice in opposite directions. Employing the U-turn technique eliminates the necessity of switching the transportation direction, and it is possible to simplify a transport control and to reduce time for scanning.
When two or more documents are automatically fed in such an image scanning device, it is controlled such that a transportation interval between the documents is constant by determining a timing for supplying a succeeding document from a paper feed tray based on a transportation position of a previous document. Specifically, it is detected that the previous document has arrived at a detection position on a transportation route using an optical sensor, and an amount of transportation of the document after the detection is measured, thereby determining a timing for supplying the succeeding document. The measurement of the transportation amount is carried out, for example, by counting the number of steps supplied to a step motor for transportation using a counter, and the succeeding document is supplied when a value of the count number by the counter reaches a predetermined value.
In the case of an image scanning device in which the transportation interval between documents is different depending on an operational state, providing such a counter makes it possible to change the transportation interval by changing the value of the count number by the counter. For example, in the case of an image scanning device employing the U-turn technique, it is necessary to increase the transportation interval in the double-side scanning in which a document is caused to pass through an image sensor twice via a U-turn path in comparison with the case of the one-side scanning in which a document does not pass through the U-turn path. Accordingly, in the double-side scanning as compared to the case of the one-side scanning, it is required to cause the counter that generates a supply timing to count a greater value of the count number.
In the conventional image scanning device, a timing at which a document has arrived at a predetermined detection position is correctly detected by detecting the document that is being transported using an optical sensor. Further, by counting a transportation amount of the document that has been detected by the optical sensor after the detection using a counter, it is possible to change a transportation interval by changing the value of the count number by the counter.
However, the measurement of the transportation amount by the counter includes an error due to slipping of a document or the like. Therefore, increasing the value of the count number by the counter also increases the error, thereby posing a problem of decreasing an accuracy of the transportation interval. Accordingly, there is a problem that, when a difference between two selectable transportation intervals in the same image scanning device increases, the value of the count number by the counter increases in a case where a longer transportation interval is selected, thereby reducing a control accuracy of the transportation interval. Further, a problem has been noted that a counter with more digits is required, thereby increasing production cost.